Lewis structures limitations

Simple second-order perturbation theory is well adapted to describe the energy lowering and occupancy shifts associated with delocalizing interactions between specific Lewis and non-Lewis NBOs. The unperturbed wave function /l corresponds to the perfectly localized Lewis structure limit, with all Lewis-type (electron donor) NBOs fully occupied and all non-Lewis-type (electron acceptor) NBOs completely vacant. The perturbative interaction of donor NBO a B with acceptor NBO CD leads to the approximate second-order energy lowering... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

Lewis, G. N., 60, 64, 398 Lewis acid, 398, 473, 671 Lewis base, 398, 473 Lewis structure, 65 writing, 67 Lewis symbol, 60 Lewis theory, limitations of, 115... 

The limitations of conventional Lewis structures are evident if we consider that the polarity of the C=0 bond is usually described by means of the two resonance structures 1 and 2. 

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

Why does the charge on the central alkyl group reduce the resonance energy of Sn2 transition states The answer is straightforward, once it is realized that the X A 1 X structure commonly contributes to the two Lewis structures. Thus, the higher the contribution of the triple ionic structure, the more similar the two Lewis structures, and the lower the resonance energy becomes. In the theoretical limit, where the X A+ X structure becomes the dominant structure in the TS, say 100% of the TS wave function, then the resonance energy should go to zero. It is seen that the semiempirical expression in Equation 6.20 mimics this limit. Other semiempirical expressions make similar predictions (5,53), but Equation 6.20 is the simplest one. 

The double arrow indicates that these Lewis structures are resonance forms. It does not mean that the molecule vibrates back and forth between these two forms but rather that there is just one form of ozone. The reason for writing the two structures is that there is a limitation in the ability of Lewis structures to describe the electron distribution in some molecules. 

So, honest disagreements exist among chemists as to the best Lewis structures for molecules that, at least at first glance, appear to exceed the octet rule. This uncertainty shows the limitations of the Lewis model with its localized electron pairs. Note, however, that even with its limitations, it is still very useful because of its simplicity. The ability to obtain a reasonable bonding picture with a back-of-the-envelope model has led to the enduring influence of the Lewis model. ... 

Carbon monoxide (CO), one of three oxides of carbon, is an odorless, colorless, toxic gas at 25°C and 1 atm. It is a by-product of the combustion of carbon-containing compounds when there is a limited oxygen supply. Incidents of carbon monoxide poisoning are especially common in the winter in cold areas of the world when blocked furnace vents limit the availability of oxygen. The bonding in carbon monoxide, which has the Lewis structure C=0 , is described in terms of sp hybridized carbon and oxygen atoms that interact to form one a and two v bonds. 

Lewis structure. A representation of covalent bonding using Lewis symbols. Shared electron pairs are shown either as lines or as pairs of dots between two atoms, and lone pairs are shown as pairs of dots on individual atoms. (9.4) Ligand. A molecule or an ion that is bonded to the metal ion in a complex ion. (22.3) Limiting reagent. The reactant used up first in a reaction. (3.9)... 

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